Micro-emulsions are thermodynamically stable isotropic dispersions of oil and water containing domains of nano-meter dimensions or bi-continuous phases, which are stabilized by the interfacial film of surface active agents. The properties of micro-emulsions make them attractive for cosmetic formulations from several different aspects. Micro-emulsions are transparent (or translucent), giving a perception of a “clean” system, and they form emulsions spontaneously. Micro-emulsions have an ultra-low interfacial tension between oil and water interface, which is key for their formation. This interfacial tension property also makes micro-emulsions potentially better (1) deep pore cleansers, e.g. in wash-off compositions and (2) follicle delivery vehicles, e.g. in leave-on compositions, due to enhanced capillary effects.
It is relatively easy to micro-emulsify short chain oils. On the other hand, bulky oils such as long chain fatty esters and triglycerides, such as those present in skin sebum, are notoriously difficult to be micro-emulsified. The interaction of such high molecular weight oils with surfactants is not well understood in the art and is radically different from that of conventional alkane oils. This is the challenge addressed by the present invention. Specifically, the problem addressed by the present invention is how to micro-emulsify bulky and high molecular weight oils, such as triglycerides in sebum. It is well known that co-surfactants can enhance micro-emulsification efficiency. But the selection of co-surfactant is mainly based on trial-and-error.
Regarding applications in cosmetics, sebum is one of the most important oils related to surfactant-oil interactions on skin. Sebum is a complex mixture of triglycerides(57%), wax esters(26%), squalene (12%), sterol esters (3%) and free sterols (2%) produced by sebocytes (cells of the sebaceous glands in the skin) and secreted to the skin surface. A frequent and undesirable skin condition is “oily skin,” the condition which results from the excessive amount of sebum on the skin. Oily skin is associated with a shiny, undesirable appearance and a disagreeable tactile sensation and affects various age groups. Therefore, cosmetic products that obviate the undesirable appearance and disagreeably tactile sensation due to sebum are highly desirable. Additionally, the present invention is based on the discovery that micro-emulsification of sebum on contact with inventive compositions according to the present invention will form micro-emulsions which can enhance (1) deep pore cleansing of skin and (2) deep pore delivery of skin benefit actives through the follicles.
Cleansing into pores presents a considerable technical challenge over and above normal surface cleaning processes due to the difficulty to access the target soil. Also, the nature of the oil is of great importance in determining both the feasibility and rate of removal. Although conventional skin cleansers, such as soap and water, are very effective in removing surface dirt and grease, continuous production of sebum by the sebaceous gland ensures that the re-greasing process begins immediately so that after a relatively short time, the sebum level is reestablished.
Micro-emulsion is an ideal medium for delivery of skin benefit agents into the skin. Applying products that are themselves micro-emulsions has been widely used in a variety of skin care products and delivery vehicles and examples of such products are described in U.S. Pat. Nos. 5,858,954 and 6,303,662, and in PCT Published Application WO 02/102327. Describing micro-emulsifying oils or soils on contact is Mondin, et al., U.S. Pat. No. 6,191,090, relating to a hard surface liquid cleaning composition based on EO—PO nonionic surfactant in combination with other components of the emulsification system described therein.
The art cited above does not suggest or disclose cosmetic compositions or methods for micro-emulsification of sebum on contact of a composition with skin, nor do they suggest or disclose an efficient emulsification system for doing so. Therefore, a need remains for novel cosmetic compositions and methods that make use of natural skin sebum to enhance skin cleansing or to deliver actives to skin from leave-on or wash-off compositions.